This invention relates to the control of the power of transmitted signals in cellular communications systems, such as cellular radio communications systems using TDMA (time division multiple access) or CDMA (code division multiple access) techniques. One example of a TDMA system is referred to as an IS-136+ system.
In a cellular radio communications system, communications take place between base stations in respective cells and remote, typically mobile, stations using spectrum sharing techniques such as TDMA, CDMA, and FDMA (frequency division multiple access). In order to enhance the operation of such systems, for example in order to limit power consumption, minimize interference among different users and/or cells, and maximize the system capacity in terms of numbers of users, it is known to limit the powers of signals transmitted by the base and remote stations to only the levels necessary at any time for effective communications. Accordingly, closed loop transmitted signal power control is known in such systems.
For example, Gilhousen et al. U.S. Pat. No. 5,812,938 issued Sep. 22, 1998 entitled xe2x80x9cReverse Link, Closed Loop Power Control In A Code Division Multiple Access Systemxe2x80x9d discloses a power control process in a CDMA system in which a base station measures SNR (signal-to-noise ratio) of a signal received from a mobile station and, in dependence upon this SNR relative to threshold values, generates power control commands to instruct the mobile station to change its transmitted signal power. Each power control command comprises two bits indicating a power xe2x80x9cturn upxe2x80x9d or xe2x80x9cturn downxe2x80x9d command. If both bits indicate a xe2x80x9cturn downxe2x80x9d command, the mobile station decreases its transmitted signal power by 1 dB. If the first bit is a xe2x80x9cturn downxe2x80x9d command and the second bit is a xe2x80x9cturn upxe2x80x9d command (referred to as a xe2x80x9cdown-upxe2x80x9d command), the mobile station does not change its transmitted signal power. If the first bit is a xe2x80x9cturn upxe2x80x9d command, the mobile station can increase its transmitted signal power by 1 dB depending upon the second bit.
While such a system enables the remote (mobile) station transmitted signal power to be increased, decreased, or unchanged for each power control command, each such command requires two bits. In an IS-136+ TDMA system referred to above, only a single bit for each time slot is allocated for fast power control purposes. In general, it is desirable to reduce the average number of bits per time slot or time period which are used for power control, while providing a process that enables fast power control for responding to rapid signal changes, for example Raleigh fading.
It is also known in a CDMA system to use a single bit for each fast power control command, this being for example a binary one to command the remote station to increase the transmitted signal power by for example 0.5 dB, and a binary zero to command the remote station to decrease the transmitted signal power by 0.5 dB. In this case successive power control commands can alternate between one and zero to maintain a transmitted signal power that is relatively constant, i.e. that changes up and down by only 0.5 dB.
While such a power control process may be acceptable in a CDMA system in which the remote station transmitted signal power can be changed in relatively small steps, it is impractical in a TDMA system in which the transmitted signal power can only be adjusted in large steps, for example of 4 dB or 2 dB for TDMA mobile terminals. Applying alternating power increase and power decrease commands to such a remote station adversely affects operation of the power amplifier of the transmitter and the transmitted signal spectrum, and additionally results in an excessive requirement for processing of power command signals. In addition, such a system is prone to errors; whereas in a CDMA system there is an outer power control loop that enables the effects of power command errors to be minimized, this is not the case in the TDMA system. Even in a CDMA system, it may be desirable to reduce the average rate at which power control bits are communicated.
Accordingly, there is a need for an improved fast power control process which for example can be used in a TDMA system using only one bit per time slot and which avoids these disadvantages.
An object of this invention, therefore, is to facilitate provision of such a fast power control process.
One aspect of this invention provides a method of producing power control bits at a first station for use in controlling power of a signal transmitted by a second station and received at the first station, comprising the steps of: determining a signal quality parameter of the signal received at the first station; in response to the determined signal quality parameter exceeding an upper threshold by an upper threshold margin, producing a power control bit with a first binary value and increasing the upper threshold margin; in response to the determined signal quality parameter being below a lower threshold by a lower threshold margin, producing a power control bit with a second binary value and increasing the lower threshold margin; and in response to the determined signal quality parameter not exceeding the upper threshold by the upper threshold margin and not being below the lower threshold by the lower threshold margin, producing a power control bit with a binary value opposite to the binary value of the preceding power control bit and setting the upper and lower threshold margins to predetermined values.
Conveniently the lower threshold margin is also set to its predetermined value in response to the determined signal quality parameter exceeding the upper threshold by the upper threshold margin, and the upper threshold margin is also set to its predetermined value in response to the determined signal quality parameter being below the lower threshold by the lower threshold margin.
The signal quality parameter preferably comprises signal-to-noise ratio of the signal received at the first station. Each increase in the upper and the lower threshold margin is preferably by an amount substantially equal to a predetermined power change step size, e.g. 4 dB, of the signal transmitted by the second station divided by an integer N greater than one. The integer N is preferably from 3 to 7, for example N=5.
Preferably the signal received at the first station is a signal received in a time slot of a time division multiplexed communications system, and the signal quality parameter of the signal is determined for each time slot of the received signal. The method preferably includes the step of transmitting a further signal from the first station, for reception by the second station, in time slots of a time division multiplexed communications system, one bit of the further signal in each time slot being constituted by the power control bit.
Another aspect of the invention provides a method of controlling, in response to power control bits produced at a first station by the method recited above, power of a signal transmitted by the second station for reception by the first station, comprising the steps of: in response to successive ones of the power control bits, producing a variable which is incremented in a first direction in response to each power control bit having the first binary value and in a second, opposite, direction in response to each power control bit having the second binary value; in response to the variable being incremented in the first direction to exceed a first threshold, decreasing the power of the signal transmitted by the second station by a predetermined power change step size and changing the variable in the second direction; and in response to the variable being incremented in the second direction to exceed a second threshold, increasing the power of the signal transmitted by the second station by the predetermined power change step size and changing the variable in the first direction.
For example, the variable can comprise a count of a counter. This method preferably includes the step of receiving each power control bit in a respective time slot of a time division multiplexed further signal transmitted by the first station.
A further aspect of this invention provides a method of controlling power of a signal transmitted between a remote station and a base station of a cellular radio communications system, comprising the steps of producing power control bits at one of said stations, and controlling power of a signal transmitted by the other of said stations, by the methods recited above.
Typically said one of said stations can be the base station and said other of said stations can be the remote station. This method can further comprise the steps of producing power control bits at said other of said stations, and controlling power of a signal transmitted by said one of said stations, by the methods recited above.
A further aspect of this invention provides a station for a cellular radio communications system, comprising a transceiver and a control unit arranged for operation in accordance with a method recited above.